Aspirators and aerators are well known. The usual form of an aspirator is based on the decrease of static pressure in a liquid flowing through a constriction in some form of venturi member, i.e. a convergent-divergent duct. Pressure is lowest in the neck of the duct.
A conventional venturi duct is defined by inwardly-curved walls. The duct is usually symmetrical about its axis.
GB-A-1365184 discloses an ejector device, comprising two tangentially-mounted ejector nozzles, adapted to rotate in a microorganism culture liquid and to create a flow of the liquid under pressure with entrained air.
U.S. Pat. No. 4,297,214 discloses aeration apparatus comprising a hollow shaft and, near the bottom of the shaft, a gas outlet and a vaned impeller. On rotation, liquid is circulated past the gas outlet and through the impeller. Tangential velocities of 200 to 450 m/min are disclosed
GB-A-2043475 discloses a hollow shaft and an impeller having tangential ribs and radial tubes adapted to draw in gas, on rotation, owing to the "liquid breakaway zones" created in the wake of the tubes. Data are given, but not on the rate of rotation.
EP-A-0155701 discloses a rotor, mounted on a hollow shaft, having radial grooves. When the shaft is rotated in a liquid, gas flows into the groovesand into the liquid as fine bubbles. A positive supply of gas is required, e.g. at a rate of 30 1 air/min and at a speed of rotation of 1000 rpm.
Other stirrers requiring a positive input of gas are disclosed in GB-A-0832526 and GB-A-2059788. An agitator comprising a prism-shaped rotor, having near radial channels discharging at the axial leading edges of the faces of the prism, is described in GB-A-0745457.